The present invention relates to cement compositions utilizing a pozzolan material, preferably fly ash. One of the principal advantages of the present invention is that it provides cement compositions which are significantly cheaper per unit volume than conventional cement compositions having essentially the same structural properties. This result is achieved through proper proportioning of the various ingredients in the cement composition and the substitution of relatively large amounts of inexpensive pozzolan material for the more expensive cementitious ingredients normally utilized.
It has been known for many years that various finely divided, silicious materials react with lime in the presence of moisture to produce cementitious material which may be mixed with sand and stone to form a product similar to modern concrete. These silicious materials, which are commonly referred to as pozzolans, occur naturally or are by-products of various manufacturing processes. Examples of pozzolan materials include blast furnace slag, volcanic ash, calcined shale, trass, pumice, diatomaceous earth, silicious clays, and fly ash, which is the fine, solid by-product contained in the gases from the combustion of pulverized coal. With the advent of increasing public environmental concerns and the resulting limitations imposed on gaseous and particulate emissions from such combustion reactions, increasingly large amounts of fly ash are being recovered from the stack gases of commercial coal burning facilities, particularly high efficiency electric power generating plants. Despite the resulting availability of considerable quantities of fly ash, at present there is no major commercial use for fly ash.
Fly ash, as well as other pozzolan materials, have been utilized to replace a portion of the more expensive cementitious ingredients conventionally utilized in various cement compositions, but full utilization of fly ash has not been achieved. The primary obstacle to the use of larger proportions of fly ash in these cement compositions is that the reaction of pozzolan materials, including fly ash, with lime is slow compared to the normal cement reaction. Thus, pozzolan containing cement compositions have an early compressive strength (typically measured at seven or twenty-eight days) which is significantly less than similar cement compositions based on conventional cementitious materials, such as Portland cement. If pozzolan is substituted for too large a proportion of the cement, the resulting cement composition will have a poor initial strength and will require additional time prior to imposition of the service load. It may be necessary to provide external support for the cement composition until the pozzolanic reaction has proceeded sufficiently so that the cement composition is self-supporting.
The slow curing time of cement compositions having a high proportion of pozzolan material is unacceptable or undesirable for most commercial applications. Attempts have been made to solve this problem by utilizing heat to accelerate the curing rate and by adding large amounts of excess lime and/or various chemicals. These techniques have produced various specialized products, but they have not accelerated the pozzolanic reaction sufficiently to be useful in preparing cement compositions suitable for a broad range of structural applications.
The present invention concerns cement compositions which realize the economic benefits of using larger amounts of pozzolan material. This result is achieved by properly proportioning the ingredients in the cement composition and by including in the composition certain alkali metal ions and, in addition, anionic constituents capable of forming complexes with ferric ions, i.e., iron complexing agents. By utilizing the appropriate amounts of cement, pozzolan, water, and fine aggregate, it is possible to minimize the void content of the cement composition and to insure maximum compressive strength. The addition of relatively large amounts of sodium, potassium, and/or lithium ions apparently accelerates the pozzolanic reaction and makes it possible to add the larger amounts of pozzolan material, in the proper proportion to the other ingredients of the cement composition, without incurring a concomitant loss in early compressive strength. The iron complexing agents assist in promoting early compressive strength by chemically tieing up ferric ions which under normal circumstances would engage in chemical reactions inhibiting the cementitious reactions necessary to the development of compressive strength.
Because these benefits can be achieved when the requisite alkali metal ions and iron complexing anions are provided in the form of sodium chloride, the present invention has the further significant advantage that the cement compositions can be prepared from sea water or other brackish waters. Prior to the present invention it was generally believed that the incorporation of sea water in cement compositions would be deleterious to the product. The present invention now makes it possible to prepare relatively inexpensive cement compositions with sea water, an advantage which is particularly useful in localities where sea water is more readily available than fresh water.